This invention relates to fuel control systems for controllably supplying fuel to a combustion engine. More specifically, this invention relates to a fuel system control wherein fuel is accurately and electronically metered to an engine independent of pumping functions.
A wide variety of fuel supply systems are known throughout the prior art. Typically, fuel supply systems of the prior art comprise a pumping device which is driven or controlled by the engine to supply fuel from a fuel tank to a fuel nozzle, such as a spray nozzle for spraying fuel into the combustion chamber of a gas turbine engine. Importantly, the fuel supply system includes some type of flow control device for quantitatively adjusting the fuel delivery in accordance with engine fuel requirements. In many systems, positive displacement pumps have been used for supplying liquid fuel to an engine since such pumps are usable to perform both the pumping and metering functions. See, for example, U.S. Pat. Nos. 2,779,291 and 3,610,782. However, positive displacement pumps experience substantial and variable fluid pressure drop which gives rise to inefficiencies in operation. For example, as pressure differential across the pump increases, pumping efficiency and metering accuracy decreases. Accordingly, positive displacement pumps serving both to pump and meter fuel have not provided the wide range fuel flow metering accuracy required in many internal combustion engines.
Some prior art fuel control systems have been proposed including separate pumping and metering devices. See, for example, U.S. Pat. No. 3,056,259. Such systems typically include one or more pumps, and a separate meter or throttling device to control fluid flow. These arrangements have nor been totally satisfactory, however, since substantial variations in pressure difference across the metering device have still been found to occur, and such variations contribute to inaccuracies of the flow system. Other prior art fuel control systems have been proposed including separated pumping and metering devices together with secondary pumps and fuel feedback controls. See, for example, U.S. Pat. No. 3,233,651. Such systems do not, however, eliminate pressure variations across the metering device, and thereby do not accurately control the metered flow over a wide flow range. Still other prior art fluid flow systems have proposed separate pumps and meters driven from a common power source, with the pressure drop across the metering device being controlled by a pressure regulation device. See, for example, U.S. Pat. No. 3,515,164. However, systems of this type are designed for applications requiring constant fluid flow, and are not well adapted for use with combustion engines requiring accurately metered fuel flows over a relatively broad operating range. Moreover, in requiring a common power source for both metering and pumping functions, these devices do not lend themselves for use with modern, highly accurate, and energy-efficient electronic control systems.
This invention overcomes the problems and disadvantages of the prior art by providing a fuel control system having independently operated pumping and metering devices, and wherein the metering device is variably and electronically controlled for fuel delivery over a wide flow range independent of system pressure changes.